Neurotransmitter regulation of bone metabolism has been a subject of increasing interest and investigation. We have analyzed skeletal structure in mice with disruption of the serotonin transporter gene (5-HTT-/- mice). 5-HTT-/-mice have reduced bone mass, size and strength compared with wild type littermates. We have also shown that osteoblasts and osteocytes possess a functional system for both responding to and regulating 5-HT activity by expressing the 5-HTT and 5-HT receptors. Finally, treatment of growing mice with selective serotonin reuptake inhibitor results in reduced bone accrual. In sum, this data suggests that the 5-HT signal transduction system may play a significant role in bone biology. We propose that 5-HT is a regulator of differentiated function in bone cells. We also hypothesize that the 5-HTT functions to regulate the ambient concentration of 5-HT in bone, and disruption of the 5-HTT leads to reduced skeletal mass. To test these hypotheses, we propose studies with the following specific aims: Specific Aim 1. Assess the differentiation paradigm of osteoblasts in ex vivo cultures from 5-HTT-/- mice Specific Aim 2. Characterize osteoclast formation and activity in ex vivo bone marrow cultures from 5-HTT-/- mice Specific Aim 3. Characterize the skeletal phenotype of mice with tissue-specific disruption of the 5-HTT gene Specific Aim 4. Characterize changes in bone mass and structure in ovariectomized mature mice treated with an SSRI. The proposed studies will allow us to define the modulatory role of serotonin on both bone formation and resorption functions ex vivo, and to better characterize the skeletal consequences of disruption of the 5-HTT gene in vivo. Characterization of the 5-HT role in bone biology may have profound implications for clinical practice, since the use of pharmacologic agents which affect the 5-HTT is widespread. This research should contribute to new treatment and prevention regimens for osteoporosis and thus have a significant positive impact on the health care of the aging population.